In Resident Evil Code: Veronica, Alexia Ashford's boss themes are all complemented by soprano vocals, which, understandably had no lyrics in the original Dreamcast release, as all the music was produced with sampled MIDIs. However, in the Darkside Chronicles, which remixes the song, replacing MIDI samples with a real soprano, the song still has no lyrics. This is actually a reference to Alexia herself: the vocals are seemingly human, beautiful, and refined, but on further inspection are inhuman, speaking but not really saying anything - Alexia's human shell covering the monster within.
It is also worth noting that both of her themes contain the music box theme that comes to represent Alfred and later Alexia, but only fragments. It would seem that these fragments of a once complete melody represent Alexia's corruption; very little of her remains by the end of the game.
Another take on Alexia's soprano theme could referencing the nature of dramatic coloratura soprano in classic operas as well. Most characters who are written as dramatic coloratura soprano are either tragic, mad, or evil. The fact that she and Alfred were "created" brings a tragic fate to both of the twins and their father, as both of the twins succumb to madness. This leads to Alexia becoming obsessed with becoming a mighty queen, a queen who is mad and evil. This is actually a good reference.
The franchise is not just a horror game series, it's progressed similarly to horror movies that have turned into action flicks. Think about it, the first game was like a B-Movie, it had a sequel that introduced darker elements and a coherent story, later had more emphasis on action, as well as remaking the first game to introduce more continuity to be consistent with the sequels, it's like how Evil Dead progressed, in a way.
Look at how utterly inefficient the First Aid spray is. Even the smallest cut or graze requires just as much as a huge gaping wound. Umbrella originally started life as a legitimate Pharmaceutical company — the First Aid spray is the greatest instance of commercialism in the Resident Evil universe and responsible for nearly everything bad that has ever happened. Umbrella had the secret to producing an aerosol capable of regenerating almost any injury, but deliberately designed it so it was both horribly inefficient and horribly expensive; explaining why there are so few cans outside of government and Umbrella facilities — they're the only ones that can buy such an expensive but revolutionary product in bulk. This made them obscenely rich and capable of affording their seemingly endless supply of research facilities and equipment. After Umbrella went bankrupt, the formula to the First Aid spray was bought out by a rival company who kept the inefficiencies but lowered the price enough to give the product mass-market appeal. By Resident Evil 4, the still limited supplies required the Merchant to put on a cap of one spray per customer. By Resident Evil 5, however, the First Aid Spray is cheap and mass-produced on the scale that allows Chris to buy as much of the stuff as he could possibly need.
As a whole, the series has moved away from Survival Horror and become more Actionized because as time goes on the characters are better prepared.
RE 0 and 1: The playable characters have their first encounters with Zombies, as they're performing their jobs on what initially appears to be routine police work. They have no idea where any of these zombies are coming from.
RE 2: Both Leon and Claire are entering Raccoon City for reasons that are unrelated to any zombie fighting. They have also never faced zombies before, but are caught in a local zombie apocalypse.
RE 3: Jill is caught in the middle of RC's local Zombie Apocalypse. She knows about the zombies, but is under equipped. She spends most of her time running from an advanced bio-weapon with far stronger resilience and power. Raccoon City is destroyed by the U.S. government, indicating that more people are now aware of the threat of bioweapons.
RE CV: When you gain control of Claire, she has just been released from a prison cell armed with a lighter, and the outbreak is well under way.
RE 4: Leon is sent in to rescue the president's daughter, and is understandably a little paranoid after the events of RE 2. After this, Leon reports back describing exactly what he found out in Spain.
RE Revelations: The playable characters now belong to organizations meant to fight all manner of BOWs and the like. Hunters were used in a terrorist attack merely a year before the main events of the game.
RE 5: A black market weapons deal is going on in Kijuju, and the player characters are sent in as backup to stop it. Though initially poorly armed, there is definitely some support infrastructure which is in place to help them out, so they can go to town.
RE 6: The whole world is in danger of getting infected with the C-Virus. The characters are more experienced then ever and actually prepared, and almost everyone in a position of power is supporting them in some capacity. Even Sherry Birkin is now able to take down just as many B.O.Ws as Leon or Chris.
RE 7: The decision to go back to the series' horror roots and remove most of the combat is justified by players taking the roles of civilians and none of the series' main protagonists, the former being in the same position that the latter were during RE 0 and 1.
RE Village: Is a noticably Actionized Sequel to 7, as Ethan has already went through the events of the last game, with his smoother and more professional reload animations and ability to push back or parry enemies being direct evidence of his greater skill and experience. Meanwhile, Chris, in his short sequence, with his expert training and access to military grade firepower, puts waves of lycans into the ground in short order.
It's a little fanwanky, but the membership of the Echo Six squad in Operation Raccoon City could be taken as implicit confirmation that the RE-verse has less restrictions on women in the military, since it features three women in a combat team. That would retroactively remove some of the problems from Jill's background, although it wouldn't help with the age issue.
Why does the knife tend to suck so much in terms of combat against zombies and other various nasties? Because that's how much good a knife would be in such a situation in real life (getting up close to monsters who could convert you into one of them with a bite is the absolute last thing you'd want to do).
The lone exception is Code: Veronica, where someone evidently remembered that a huge, heavy, razor-sharp blade can effectively cripple a zombie by severing tendons and muscles. Other games where the knife is more useful tend to be against enemies that aren't nearly as indifferent to pain as zombies are.
Why is there a noticeable drop in Rebecca Chambers' abilities between RE 0 and RE 1? When RE 0 starts, it's night. When it ends, it's dawn. When RE 1 starts, it's night again. When RE 1 ends, it's dawn. It's likely the poor girl was starting to suffer from serious starvation, thirst, and sleep deprivation to such a degree that being functional at all takes a big kind of badass.
Military and police, even at Rebecca's Improbable Age, are often pushed to be active for long periods of time, and the S.T.A.R.S. unit likely was inclined to have gone through significant physical and mental training courses in the past given how Wesker sees them fit enough to use against the Tyrant for training data. Rebecca lasting upwards to 48 hours isn't too implausible as a result - but having to deal with the Leech Queen and the number of horrors leading up to the Spencer Mansion, along with witnessing numerous horrific sights and deaths along with her impromptu partner Billy Coen having gone off on his own path to never be seen again? Even if you consider the idea that she's managed to deal with hunger and thirst, anyone would be worn down from two long days of hardcore survival against inhuman threats back-to-back.
Why does Piers Nivans gain an arm capable of producing electricity when he infects himself with the C-Virus? Because that's what happens when you combine the T- and G-viruses. The C-Virus is made of the t-Veronica virus, a variation of the T-Virus, and the G-Virus variant found in Sherry Birkin's body. Thus, a similar thing could happen, as the t+G virus first appeared in Resident Evil: Dead Aim.
Also, as for why it generates electricity: one of the files in the series mentions that the T-virus destroys cells (causing rot) in order to produce a lot of energy, which is used for the reanimation. The G-virus gives extreme, but hardly controllable, regeneration. Combine the two, and you get a being able to shoot lighting and able to easily regenerate from wounds and the strain induced to produce said energy (in fact, mutated Piers can regenerate, and his thunderbolts are Cast from Hit Points). Though, it's an unstable equilibrium: if T prevails, we have a weaker T monster without G regeneration (the tentacled Tyrant); if G prevails, the subject tends to self-destruct (Duvall).
One of Ada's signature items in the later games is a Grappling-Hook Pistol that she uses liberally and in blatant view of pretty much everybody. The single exception to this? RE2, her first appearance, where next to nothing was known about her by any party. How could she possibly have survived that drop into the bottomless pit? Her grappling hook pistol! And she didn't bother to try concealing that she had it afterwards because by that point everyone would have realized she possessed something to that effect in order to be able to walk away from a drop like that.
Even though Leon B is the canon plot direction (where Ada doesn't fall to her "death"), this still can explain how she was able to return to throw the rocket launcher to Claire in her B game.
All of the complicated puzzles found in the series (until Resident Evil 4) are part of the Umbrella Corp's security protocols. Think about it: if your company is developing the deadliest biological agents known to man, you'd want to keep that stuff locked up tight. In order to prevent theft, sabotage, or escape of one of the test subjects, the Umbrella Corp has set up a complex procedure for totally locking/unlocking their facilities, part of which involves that someone be a skilled piano player. In the event that something goes wrong, all of the doors are locked and their keys are secured inside either a puzzle or a specific room. They probably assigned each key/puzzle to a specific person or a select few people so that no single person would fully know how to bypass their security. Fortunately, some of them left clues for others to find in case things went from bad to worse. Guess the Umbrella Corp didn't do a good enough job of screening for employees that wouldn't leak company secrets.
Many have mocked the unnecessary boulder punching in Resident Evil 5, and it may have indeed not been a very practical choice of action. We all know that Chris rightly has issues regarding comrade mortality and during Resident Evil 6, Chris has many, many situations that put his comrades in harm's way, which really stresses him out... What does he do whenever one of those situations arise? He hits whatever he perceives as an obstacle. Whether it does any good or not, whether it be an escape pod, metal bars, or a boulder. Turns out it's a bit of a habit for him in desperate situations. What kind of situation was he in when he punched that boulder?
Since injecting himself, Wesker has become nigh-indestructible. He evidently does not wish to stick around the mansion when it is set to explode, backs away from his fight with the pyrokinesis-wielding Alexia as soon as he notices Chris is available to do it for him, and is finally destroyed in a volcano. Heck, he even exposes himself to Uroboros, which could be destoyed by pretty much nothing but fire. Kind of a theme going on?
Regarding the volcano, a certain irony that a man who spent his whole life making a mockery of nature and in effect becoming a mockery of nature, was destroyed by one of the most powerful things nature had to offer on the planet he was seeking to infect. (Okay, and rocket launchers, but still.)
William Birkin spent the last part of his life working on the G-Virus, insisting that it was his ultimate creation that could bring about a perfect lifeform if Umbrella gave him time to perfect it. Cut to Resident Evil 6 and Sherry, his daughter and the only person to be infected by a controlled, pure form of G, is a (seemingly) unaging human with a powerful Healing Factor and absolutely no negative side effects. By comparison, even Wesker required frequent injections to remain stable. Looks like Umbrella should have listened to their top scientist after all!
Taking into account All There in the Manual, it's clear why Leon kicks so much more ass in 4 than in 2: The reason he's late to his job in 2 is because he was up all night drinking so he spent the Zombie Apocalypsehungover. Resident Evil 4 is what happens when Leon's sober and ready and he's more agile, more physical, and stronger all around (and more extensively trained as a U.S. Government Agent, but that's neither here nor there).
He's also given up smoking recently. Think about it: he's carrying gum, which former smokers carry to ward off nicotine cravings, although this in and of itself isn't a sign. However, he immediately offers gum to Luis when he asks him for a smoke, a jump most people wouldn't make unless that's what they use gum for.
With both Resident Evil: The Darkside Chronicles and Resident Evil 6 delving into the effects of viruses on more fortunate individuals alongside less fortunate ones, it becomes clear that those who are frightened or in fear become the abominations and monsters throughout the series, as well as zombies and the contamination can just spread from there. Those who are particularly vengeful or have controlling / wannabe god mindsets and filled with at least some sort of acceptance for their infection gain some sort of monstrous factors while still maintaining their sentience. And those who can accept and come to terms with their situation altogether but maintain Heroic Willpower and a steadfast mindset in accepting both life and pain all the same can use the virus for good, thus Manuela's and Piers' more beneficial transformations compared to the norm.
And even if you've become an uncontrollable and insane abomination hellbent on destroying everything around you, in some cases they maintain their sentience - particularly the Ja'vo forces in 6, and implied with Mutated Deborah when she merely pushes Helena to the side to get her out of the way rather than kill her outright.
The idea that the mutations caused by Progenitor viral strain infections are in part determined by the psychology of the host are made explicit in a file you can unlock in the "Wesker in Russia" levels in Resident Evil: The Umbrella Chronicles. Fast forward to Resident Evil: Revelations 2, and you have the T-Phobos virus, which was literally created to study and exploit this facet of the Progenitor virus.
By coincidence (or possibly not), Leon's roles in the three CGI movies actually run parallel to the three games he's appeared in.
2 and Degeneration has Leon team up with Claire to stop a small-town zombie outbreak. The final monster faced here is a human who infected himself with the G-virus and seeks to mate with a close female relative, which the duo must protect.
4 and Damnation are primarily solo missions where he investigates a Plagas outbreak in a European country.
6 and Vendetta have Leon and Chris cooperate to stop a viral outbreak in a major city.
At the end of the Resident Evil 7 Beginning Hour, if you manage to escape the house and report the police what happened no one believes it because you have LSD in your system. While this could be interpreted as the character using the drug in the past or that everything was a hallucination early on, after the release of the main game and the reveal that the monsters are created by a fungus it makes sense he would show up with LSD in his system even if he never touched it. The reason for this is that LSD was originally created from chemicals found in certain fungi. Though this does bring up the worry that if he had enough in his system to create a false positive, would that mean he's already infected?
Wesker made some notes on the possibility that the viruses cause different mutations based on the personality/psychology of the infected person, and wonders "Where does that leave me?" The answer? Vladimir was Axe-Crazy, obsessed with pain and suffering to the point of cutting himself with his knife and Licking the Blade, and becomes one of the most deformed monsters in the series; Alexia was obsessed with ants, seeing herself as a "queen", and gained ant-like traits...yet out of all the characters transformed by a virus, Wesker's appearance and personality stays the most similar to how he was before being infected. Why? Because Wesker's biggest obsession is...himself. Everything Wesker has done throughout the series has been motivated by personal gain, or simply by arrogance and A God Am I. Wesker's appearance and mind were unchanged (except for some Glowing Eyes of Doom, possibly showing how he wanted everyone to know he was different and "better" than them) because Wesker saw himself as perfect.
It's almost certainly an accident, but the revelation in Resident Evil 5 about Spencer's whole plan does a lot to explain the Tyrant. A lot of fans have joked about how the "ultimate life form" is a bald, expressionless homicidal maniac with no personality, but Umbrella was already working on reasonably effective implanted control mechanisms for it by Outbreak. It is, in short, the basis for a near-perfect soldier, which dovetails neatly with Spencer's planned eugenics program.
It's said outright in the second Wesker's Report that the Arklay mansion lab was in the worst possible place to research the T-Virus, since it was in the middle of a major forest preserve. The original outbreak in the first game thus heavily contaminated the entire forest with a virus that mutates everything it touches into a carnivorous killer. In the various Raccoon City games, it spread into a major metropolitan area and there were mild cases in the city as early as three months before the disaster, which gives it ample time to spread outside of Raccoon. Dead Aim sees a contaminated ocean liner packed with dead and undead T-Virus carriers sink in the middle of the ocean, Darkside Chronicles has an outbreak in rural South America in the middle of the rain forest, and in Revelations, there are still contaminated and potentially lethal T-Abyss mutants washing up on the shores of the Mediterranean Sea a full year after the Terragrigia Panic. Combine that with the multiple listed bioterror outbreaks mentioned in Degeneration, and it becomes obvious that the RE 'verse may not be in the middle of a full-blown zombie apocalypse in the classic style, but they are heading toward a massive mutation that could potentially affect the entire planetary biosphere.
Worse yet, the plots of 5 and 6 as well as the Revelations games constantly bring up the fact that bioterror is on a global rise, with the implication that any shady PMC, dictatorship or dealer can get their hands on black market viruses and promptly cause incidents that wipe out entire small countries. It happens in the CGI movies, and even in spinoff manga and side materials. While the constantly-corrupt pharmaceutical organizations that keep releasing massive outbreaks that kill hundreds of thousands, potentially even millions, of people are a big threat, the world is being overrun by people that have effectively weaponized bioterror as a common tactic. The world of Resident Evil certainly fits its overseas title, Biohazard, because not a year can seem to go by in canon without some outbreak.
In Resident Evil 2, if you reach the police department without picking up any objects in normal mode, you'll see a zombie in a yellow vest and camouflage pants. This is Brad Vickers. He was the helicopter pilot in the first game. The last time you saw him, he was alive and well.
It still counts for people playing the game for the first time, since it has unfortunate implications for his teammates. After all, if Brad's dead, where are everyone else? A file later reveals Chris, Jill, and Barry are in Europe dealing with Umbrella's HQ (this is then retconned bafflingly into just Chris and Barry to make way for Resident Evil 3), but that still leaves us with our next question:
Where's Rebecca? She's not mentioned in any files post Resident Evil until Resident Evil 2 for the N64 added a file tying into the as-then unreleased (and planned for the N64) Resident Evil Zero which more or less summarizes that game. Still, this is just a port of the game outside the series proper (although the files are still canon, as they also allude to Code Veronica), meaning the next mainline reference to Rebecca wouldn't come until Resident Evil 5, a full ten years after the first game. Since only Chris, Jill, and Barry are mentioned in the supplementary files in RE2, one could be forgiven for assuming Jill's scenario is canon, and Rebecca either didn't survive the Mansion...or Chris wasn't around to meet her and she was left behind.
Even though it's canon she survived the mansion (both in Resident Evil 5 and Umbrella Chronicles), there's no evidence she survived the destruction of Raccoon City, since she never linked up with Jill, Leon, and Claire, the cast of Outbreak, or even the Echo Six squad (and further, she's not even on the Wolf Pack's shitlist, just Chris, Jill, and Barry again), meaning it would've been very possible for her to have been left behind again in Raccoon City. Talk about bad luck.
There's two evidences that show that Rebecca survived. The canon Biohazard Stage Show and Vendetta. Otherwise, she would not even appear in those. Also, it was stated that Rebecca left before the Raccoon City incident happened and joined the BSAA at some point. While there is no proof that Rebecca was not in Raccoon City, there's also no proof that Rebecca was in Raccoon City, aside from the S.T.A.R.S. department, of course.
Resident Evil: Vendetta pretty much settles it, showing that she took on a non-combat role after the mansion incident, and has been devoting her time to researching Umbrella's viral agents and developing countermeasures.
In the second movie, there is a scene where Jill Valentine and Angela Ashford are hiding from zombified dogs in a school's kitchen. Jill turns on all of the stoves, and runs out with Angela. Awaiting them outside is Alice ready to blow up the gas. This is all fine and dandy until you realize that Alice has no reason to be waiting for them, and certainly no reason to be conveniently smoking the cigarette (Which she got from where?) that she used to blow up the gas. She even has a fireproof blanket to cover up Angela with.
The fireproof blanket you see Alice pull out of a nearby case on the wall, she could of got the cigarette from Jill (she is shown smoking in the tram), Alices powers may explain why she was waiting, she might of been able to smell the gas from a distance.